Hydrocarbon mixtures having an end boiling point over 550° C. are generally not processed directly in a pyrolysis reactor to produce olefins, as the reactor cokes fairly rapidly. While limiting reaction conditions may reduce the fouling tendency, the less severe conditions result in a significant loss in yield.
The general consensus in the art is that hydrocarbon mixtures having a wide boiling range and/or hydrocarbons having a high end boiling point require an initial separation of the hydrocarbons into numerous fractions, such as gas/light hydrocarbons, naphtha range hydrocarbons, gas oil, etc., and then cracking each fraction under conditions specific for those fractions, such as in separate cracking furnaces. While the fractionation, such as via a distillation column, and separate processing may be capital and energy intensive, it is generally believed that the separate and individual processing of the fractions provides the highest benefit with respect to process control and yield.